This application is based on and hereby claims priority to German Application No. 101 49 553.6 filed on Oct. 8, 2001 and European Application No. 011 23 993.6 filed on Oct. 8, 2001, the contents of which are hereby incorporated by reference.
The invention relates to a system for phase trimming of N feeder cables, which are used for driving an antenna system, by a transmission pilot tone.
Radio communications systems use antenna systems with individual antennas which are driven via feeder cables. These feeder cables influence the polar diagram of the antenna system as a result of mechanical length differences and phase differences between them, for which reason the phase differences must not exceed maximum values, which are predetermined on a system-dependent basis.
In the case of a phased array antenna system which comprises N individual antennas and is used in so-called switched-beam radio communications systems an Nxc3x97N Butler matrix, for example, is connected upstream in order to drive the N individual antennas. A total of N feeder cables are arranged between the Butler matrix and a transmission device, and are intended to be essentially of the same length, for example for specific applications. A maximum permissible phase difference of xc2x15xc2x0, by way of example, is required, as a typical value, between the individual feeder cables.
The lengths of the feeder cables are in this case normally trimmed before the antenna arrangement is brought into use, in such a way that a network analyzer is first of all used to determine any phase difference between the individual feeder cables, with respect to one feeder cable which is used as a reference cable, and the phases are then trimmed to a standard phase by appropriate shortening of the individual feeder cables. A part of the feeder cable on a basic length is advantageously in the form of a so-called jumper cable, whose basic length is preferably used for phase trimming.
The feeder cable phases are preferably trimmed in situ, since the electrical lengths of the feeder cables vary during installation on site, due to bends in the feeder cables. Once the radio communications system, or its antenna systems, has or have been successively commissioned, there is no longer any provision for checking the phase trimming or the phase difference in detail, and this process cannot be carried out during operation of the antenna system.
The network analyzer, which is used for example by an installation team to determine the phase difference, is normally a relatively expensive laboratory item and is suitable only to a restricted extent for commissioning on site, due to its weight, its mechanical dimensions, and due to its sensitivity to environmental influences.
One possible object of the present invention is therefore to allow simpler phase trimming of feeder cables in an antenna system, without complex test equipment and without any restrictions relating to the time at which the measurement is carried out, the time taken to carry out the measurement, and the operating condition of the antenna system.
According to one aspect of the invention, a transmission pilot tone is input with a time offset, that is to say successively, into each individual feeder cable, and is output again as a received pilot tone after in each case passing through the appropriate feeder cable. The comparison between the transmission pilot tone and the received pilot tone makes it possible to determine phase differences between the feeder cables and to correct for these phase differences as appropriate by a trimming device which is connected upstream of the feeder cables.
The system allows phase trimming both before commissioning and during operation of the antenna system, and is advantageously carried out by a trimming device, which can be operated externally by a servicing team.
A phase control element for phase trimming is in this case provided in the trimming device, for each individual one of the total of N feeder cables. In one advantageous development, these N phase control elements are in the form of differential rotary capacitors, which can operated externally, so that there is no need for complex electrical driving of the phase control elements.
Coarse trimming of the lengths of the feeder cables and the fitting of waterproof connectors at both ends are advantageously carried out in the factory, while only fine trimming of the phase difference is now carried out on site, by the phase control elements. The already described problem of moisture ingress is avoided, and costs and labor time are saved.
The phase differences are determined via a serial interface by a commercially available laptop, which acts as a local maintenance terminal (LMT).
The system advantageously allows the phase differences between the feeder cables to be determined at different frequencies, thus resulting in an improvement in the accuracy of the phase trimming.
The transmission pilot tone which is input into the individual feeder cables in this case satisfies the criteria, as defined in the ETSI specifications, for so-called spurious emission of a carrier frequency, since the transmission pilot tone is in fact likewise passed to the antenna system for transmission.
The carrier frequencies which are used for the transmission pilot tone are advantageously slightly below a lower frequency band limit, which is predetermined as a function of the system, or are slightly above an upper frequency band limit. This offers the advantage that no carrier frequencies that are used for transmissions are located in the vicinity of these carrier frequencies, but at most intermodulation products. In addition, duplex filters which are used as receiving and transmission bandpass filters do not start to produce attenuation in this frequency range.